Abstract: A delicately balanced but normally closed polished ball bearing sensor highly responsive to physical vibrations of its immediate environment having security significance. A ball bearing is confined within a cylindrical metal shell with a very slight clearance permitting free vibration of the ball within said shell and resting by gravity upon the rounded head of a metal electrode mounted axially in an epoxy block also confined within said shell, conductor leads being soldered to said shell and said electrode and the assembly being enveloped co-axially within an epoxy casting having a flat face covered by a waterproof pressure adhesive serving to secure the sensor to any smooth flat vertical surface whereby the sensor is mounted in upright vertical position.